In surgical and other medical procedures it is frequently necessary to aspirate liquids from the body of the patient. For example, in the case of a patient having a peritoneal infection, it is common procedure to aspirate liquid from the peritoneal cavity. In this and similar cases, it is desirable if not essential to assure that the liquid withdrawn from the patient's body be blocked from entrance to the suction pump or other aspirator. Without such blockage the liquid entering the aspirator can do damage to the aspirator mechanism and, what is worse, any microorganisms in the liquid can not only contaminate the aspirator but also contaminate the atmosphere by way of the exit of such microorganisms from the exhaust port of the aspirator. It is known to fit the inlet conduit of the aspirator with a hydrophobic filter which, because it is hydrophobic, functions to allow the passage of gas but to block the passage of liquid to the aspirator. Further, to assure that no microorganisms reach the aspirator in the air which passes through the hydrophobic filter, it is known to use a hydrophobic filter of sufficiently small pore size to function as a barrier to the passage of the microorganisms through the filter.
In the functioning of such a system it generally occurs over a period of operation that there is an accumulation of liquid in the filter device at the inlet surface of the hydrophobic filter. After continued accumulation, the amount of liquid can be such as to block or at least interfere with the required passage of air through the hydrophobic filter. When this occurs it is necessary to replace the hydrophobic filter device with a fresh one, or at least to empty the device of its accumulated liquid. The latter is disadvantageous in that the liquid is often highly contaminated with microorganisms and hence the emptying of the liquid and reinsertion of the device into the system presents the hazard that the liquid, in the course of being emptied from the device, will give rise to contamination of the ambience by spillage or otherwise. Further, even where the hydrophobic filter device is removed and replaced with a fresh one, if there is any substantial amount of liquid accumulation in the removed device there is the possibility of spillage contamination of the ambience during the removal and replacement procedure.
Hence, while a hydrophobic filter does assure against contamination of the aspirator or the ambience during the period of the efficient function of the hydrophobic filter, there remains the possibility of contamination of the ambience after there has been an accumulation of liquid in the device sufficient to require its replacement or repair.
It is well known that a hydrophilic filter allows the passage of air therethrough until it is saturated with liquid but blocks or at least substantially restricts the passage of air when it does become saturated with liquid. Where the pressure differential across the hydrophilic filter does not exceed the bubble point of the filter (i.e. the pressure required to force air through the filter when it is saturated with liquid), the passage of air is completely blocked when it becomes saturated. But even where the pressure differential does exceed the bubble point, the hydrophilic filter when saturated will nevertheless substantially restrict the passage of air.